In the manufacture of integrated circuits, after the individual devices such as the transistors have been fabricated in and on a semiconductor substrate or wafer, they must be connected, or “wired”, together to perform the desired circuit functions. This interconnection process is generally called “metallization” and is performed using a number of different photolithographic, deposition, and removal processes to create contacts to the transistors, wire/channels to the contacts, and vias interconnecting the channels where there are more than one level of channels.
There are a number of different metalization techniques, but generally, a device dielectric layer is deposited over the transistors, openings are formed through the device dielectric layer down to transistor junctions and gates, and the openings are filled with a conductive metal to form contacts.
In one technique called the “single damascene” or “single inlaid” process, the formation of the first channels starts with the deposition of a thin first channel stop layer over the device dielectric layer. The first channel stop layer is an etch stop layer which is subject to a photolithographic processing step which involves deposition, patterning, exposure, and development of a photoresist, and an anisotropic etching step through the patterned photoresist to provide openings to the contacts. The photoresist is then stripped.
A first channel dielectric layer is formed over the first channel stop layer. Where the first channel dielectric layer is of an oxide material, such as silicon oxide (SiO2), the first channel stop layer is a nitride, such as silicon nitride (SiN), so the two layers can be selectively etched. The first channel dielectric layer is then subject to further photolithographic process and etching steps to form first channel openings in the pattern of the first channels. The photoresist is then stripped.
An optional thin adhesion layer is deposited on the first channel dielectric layer over the entire semiconductor wafer and lines the first channel openings to ensure good adhesion of subsequently deposited material to the first channel dielectric layer. The adhesion layer is a metal such as tungsten (W), titanium (Ti), or tantalum (Ta).
High conductivity metals, such as copper (Cu), diffuse easily through dielectric materials such as silicon oxide. This diffusion can result in a conductive buildup and cause short circuits in the integrated circuits. To prevent diffusion, a diffusion barrier is deposited on the adhesion layer. For copper conductor materials, the diffusion barrier layer is composed of materials such as tantalum nitride (TaN), titanium nitride (TiN), or tungsten nitride (WN).
However, these nitride compounds have relatively poor adhesion to copper and relatively high electrical resistance so they are problematic. For simplicity, the adhesion and barrier layers are sometimes collectively referred to as a “barrier” layer herein.
For conductor materials, such as copper and copper alloys, which are deposited by electroplating, a seed layer is deposited on the barrier layer and lines the barrier layer in the first channel openings to act as an electrode for the electroplating process. Processes such as electroless, physical vapor, and chemical vapor deposition are used to deposit the seed layer.
A first conductor material is electroplated on the seed layer and fills the first channel opening. The first conductor material and the seed layer generally become integral, and are often collectively referred to as the conductor core when discussing the main current-carrying portion of the channels.
A chemical-mechanical polishing/planarization (CMP) process is then used to remove the first conductor material, the seed layer, and the barrier layer above the first channel dielectric layer so the materials and layers are coplanar with the dielectric layer. The CMP process leaves the first conductor “inlaid” in the first channel dielectric layer to form the first channels. When a thin dielectric layer is placed over the first channels as a final layer, it is called a “capping” layer and the single damascene process is completed. When the layer is processed further for placement of additional channels over it, the layer is a via stop layer.
In another technique called the “dual damascene” or “dual inlaid” process, vias and channels are formed at the same time, generally over a completed single damascene process series of first channels. Effectively, two levels of channels of conductor materials in vertically separated planes are separated by an interlayer dielectric (ILD) layer and interconnected by the vias.
The initial step of the dual damascene process starts with the deposition of a thin via stop layer over the first channels and the first channel dielectric layer if it has not already been deposited as a capping layer. The via stop layer is an etch stop layer which is subject to photolithographic processing using a photoresist and anisotropic etching steps to provide openings to the first channels. The photoresist is then stripped.
A via dielectric layer is formed over the via stop layer. Again, where the via dielectric layer is of an oxide material, such as silicon oxide, the via stop layer is a nitride, such as silicon nitride, so the two layers can be selectively etched. The via dielectric layer is then subject to further photolithographic process using a photoresist and etching steps to form the pattern of the vias. The photoresist is then stripped. An etch stop layer between the via and the second channel opening may be optional depending upon the manufacturing process being used.
A second channel dielectric layer is formed over the via dielectric layer. Again, where the second channel dielectric layer is of an oxide material, such as silicon oxide, the via stop layer is a nitride, such as silicon nitride, so the two layers can be selectively etched. The second channel dielectric layer is then subject to further photolithographic process and etching steps to simultaneously form second channel and via openings in the pattern of the second channels and the vias. The photoresist is then stripped.
An optional thin adhesion layer is deposited on the second channel dielectric layer and lines the second channel and the via openings.
A barrier layer is then deposited on the adhesion layer and lines the adhesion layer in the second channel openings and the vias.
Again, for conductor materials such as copper and copper alloys, a seed layer is deposited on the barrier layer and lines the barrier layer in the second channel openings and the vias.
A second conductor material is electroplated on the seed layer and fills the second channel openings and the vias.
A CMP process is then used to remove the second conductor material, the seed layer, and the barrier layer above the second channel dielectric layer to form the second channels. When a layer is placed over the second channels as a final layer, it is called a “capping” layer and the dual damascene process is completed.
The layer may be processed further for placement of additional levels of channels and vias over it. Individual and multiple levels of single and dual damascene structures can be formed for single and multiple levels of channels and vias, which are collectively referred to as “interconnects”.
The use of the single and dual damascene techniques eliminates metal etch and dielectric gap fill steps typically used in the metallization process for conductor metals such as aluminum. The elimination of metal etch steps is important as the semiconductor industry moves from aluminum (Al) to other metallization materials, such as copper, which are very difficult to etch.
From a reliability perspective, one of the most critical areas in a Cu dual-damascene (DD) structure is the bottom of the vias where two metal levels meet. In the typical manufacturing sequence, after the via is opened to the underlying metal conductor, it will undergo a wet clean process to remove polymers and other impurities generated during etch. Thereafter, it will undergo a degas process, such as an argon (Ar) sputter and hydrogen (H2) reactive pre-clean prior to formation of the barrier metal, Cu seed deposition and Cu plating. These pre-clean processes are designed to remove Cu oxide and etch residues (if any) from the via bottom. Insufficient pre-clean will result in increased via resistance, reduced yield and degraded interconnect reliability.
On the other hand, an aggressive sputter pre-clean process leads to loss of the critical dimension (CD) of the metal conductor, and contaminates the via sidewalls leading to poor adhesion between the barrier metal layer and the inter-metal dielectric (IMD) layer, and forms voids at the bottom corners of the via. Also, reactive pre-clean processes have limitations such as their reactivity with a wide range of contaminants such as carbon, fluorine, Cu oxide and etch residues which all have very different etch chemistries. It also reacts with constituents of a low-k IMD layer leading to loss of CD and increased k value. Thus, the extendibility of existing sputter and reactive pre-clean processes for future interconnect applications is questionable.
One proposed solution, referred to as a barrier first approach, skips the pre-clean processes by selectively removing the deposited barrier metal and contaminants only from the via bottom. However, these selectively removed contaminants are re-deposited on the barrier metal at the via sidewalls prior to deposition of the seed layer and electroplating of the metal, which poses some of the same reliability concerns.
Solutions to these problems have been long sought but prior developments have not taught or suggested any solutions and, thus, solutions to these problems have long eluded those skilled in the art.